Non-thermionic electron emissive tube comprising a ceramic heater substrate

ABSTRACT

A non-thermionic electron emissive tube of the type comprising an evacuated envelope, an electron emissive cathode assembly in the envelope, and a collector anode for electrons emitted from the emissive layer. The cathode assembly comprises a thin ceramic substrate. On one face of the substrate is a non-thermionic cathode. On the opposite surface is a heater pattern of resistive metallizing.

United States Patent 11 1 McDonie et al.

[ Dec. 4, 1973 NON-THERMIONIC ELECTRON EMISSIVE TUBE COMPRISING ACERAMIC HEATER SUBSTRATE [75] Inventors: Arthur Frederick McDonie;Richard Dale Faulkner; James Lee Rhoads, all of Lancaster, Pa.

[73] Assignee: RCA Corporation, New York, N.Y.

22 Filed: May 17, 1972 1211 Appl. No.: 254,259

3,066,236 11/1962 Sandbank 313 304 x 3,330,991 7/1967 Lavine ct al.313/346 R x FOREIGN PATENTS OR APPLICATIONS 1,432,317 5/1965 France313/337 281,761 3/1966 Australia 313 270 Primary Examiner--DavidSchonberg Assistant Examiner-Paul A. Sacher AztomeyGlenn H. Bruestle andDonald S. Cohen [57] ABSTRACT A non-thermionic electron emissive tube ofthe type comprising an evacuated envelope, an electron emis sive cathodeassembly in the envelope, and a collector anode for electrons emittedfrom the emissive layer. The cathode assembly comprises a thin ceramicsubstrate. On one face of the substrate is a nonthermionic cathode. Onthe opposite surface is a heater pattern of resistive metallizing.

8 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION The inventionrelates to non-thermionic electron emissive tubes.

Non-thermionic electron emissive tubes generally contain an electronemitting surface, or cathode which need not be heated for operation. Thecathode commonly is a layer of semiconductor material, the surface ofwhich is cesiated by application thereon of a workfunction-reducinglayer of cesium or cesium and oxygen.

For cathodes of the type in which the semiconductor is silicon or acompound or alloy of the elements of Groups IIIA and VA or IIB and VIAof the Periodic Chart of the Elements, the process for applying thework-function-reducing layer includes heating the cathode layer to arelatively high temperature, on the order of 400C to 600C, after thecathode layer has been mounted in the tube, but before the work functionreducing layer is applied thereon.-Such cathodes, and activationprocedures for them, are described for instance in U.S. Pat.'No.3,630,587 issued to Garbe on 28 Dec. I971, U.S. Pat. No. 3,632,442issued to Turnbull on 04 Jan. 1972, and U.S. Pat.'No. 3,644,770 issuedto Bell on 22 Feb. 1972. As indicated in Garbe et al. and Turnbull, theheating of the semiconductor material of the cathode at elevatedtemperatures, approximating 300C., prepares the surface of thesemiconductor by removing impurities from its surface prior to applyingthe work function reducing layer thereon.

One present means for providingthe'heating is by a resistance heatedfilament wire situated in closeproximity to the back side of a metalsubstrate on which the semiconductor layer is disposed. When thefilament is heated,'radient energy therefrom heats the metal substrateand the cathode layer. Another present means for the heating is the useof a focussed external intense light source, such as a high intensityquartz lamp in conjunction with a parabolic mirror. The light isfocussed through the transparent tube envelope directly onto the cathodematerial. Both of these approaches utilize transfer of heat by radiantenergy, and result in substantial losses of heat to other structures ofthe tube. For example, in photomultiplier tubes, some radiant energygiven off by the filament can pass by the cathode substrate as strayradiation and heat nearby dynodes so that they are damaged. Also, someof the radiant energy is reflected from the metal substrate of thecathode to other components. Similarly, the focussed light from theexternal light source is reflected on passing through the tube envelope,and further reflected from the cathode layer itself to other internaltube components. When the dynodes having an antimony layer, for latercesiation to cesium antimonide, are heated during the heating of thecathode, the antimony evaporates from the surface onto other portions ofthe tube envelope, thus degrading the quality of the tube.

Another difficulty with present heating means is that the heating isoften nonuniform. This results in nonuniformities in the characteristicsof the activated cathode.

SUMMARY OF THE INVENTION In the novel tube, a cathode assembly isprovided comprising a thin ceramic substrate on which a cathode 2 isdisposed. A heater pattern of metallizing is provided on the substratein direct thermally conducting contact with the cathode.

' With the novel structure the cathode is heated by thermal conductionrather than by radiation. The direct thermal contact provides much moreefficient heat transfer between the heater and the cathode. Therefore,the heater need not be heated to temperatures so high as to result indamaging stray radiation to other tube components. Also, the heating ofthe cathode layer is uniform. After activation of the cathode, theheater pattern becomes a passive structure, since the non-thermioniccathode need not be heated for operation.

BRIEF DESCRIPTIONOF THE DRAWINGS FIG. 1 is a side view of aphotomultiplier tube in accordance with the preferred embodiment of theinvention.

FIG. 2 is a sectional view, on an enlarged scale, of the tube of FIG. 1through a plane transversely through the tube as shown by the sectionline 22=in FIG. 1.

FIG. 3 isa plan view of one face of the cathode substrate of the tube ofFIGS. 1 and 2.

FIG. 4 is a plan view of the opposite face of the substrate of FIG. 3.

' PREFERRED EMBODIMENT OF THEINVENTION In the preferred embodiment ofthein vention, a photomultiplier tube 10 shown in FIGS. 1 and 2,includes a photocathode assembly 12 in accordance with the invention.The tube 10 includes a glass envelope 14, a number ofcesium-antimonide-coated electron multiplying dynodes 16, a number offield electrodes 18, and an anode 20 for collecting multiplied electronswhich travel from one dynode 16 to another generally along the pathindicated by the dashed lines 22. Also included in the envelope l4,but'not shown in the drawings, are sources of cesium and oxygen foractivating the photocathode 12 structure.

The photocathode assembly 12 is shown in more detail in FIGS. 3 and 4.Referring now to FIG. 3, the assembly 12 comprises a thin rectangularwafer 24 ofaluminum oxide (A1 0 about 1 cm (centimeter) wide, 3.5 cmlong, and 0.5 mm thick. One surface of the wafer is provided with arectangular pad 26 of molybdenum metallizing about 25 pm thick, which isapplied by screen printing and firing a molybdenum-steatite metallizingink commonly used for metallizing ceramics. A lead portion 28 of the pad26 extends to a contact-fastening hole 30 at the base end of the wafer24. Over the pad 26 is a thin photocathodelayer 31 of vapor-phase-grownpolycrystalline gallium arsenide phosphide between about 5pm and 30 umthick containing about percent gallium arsenide. Details ofvaporphase-growth are described, for instance, in U.S. Pat. No.3,218,205 issued 16 Nov. 1965 to Ruehrwein.

Referring now to FIG. 4, the opposite face of the wafer 24 is providedwith a zig-zag pattern of molybdenum about 25 pm thick to form aresistance heater strip 32 in contact with the wafer 24. Each end of thenearly entirely across the width of the wafer 24. These apertures 36, 38provide heat dams to improve the uniformity of heating by minimizing endheat losses. Also, to further improve the uniformity of the heating, theheater strip 32 is narrowed somewhat at portions 40, 42 near the heatdams 36, 38. The purpose of this is to increase the heat output from theheater in these regions to compensate for heat losses which occur at thetop and bottom ends despite the heat dams.

Owing to the described arrangement of the cathode assembly 12, heatingof the photocathode layer 26 is by direct thermal conduction. Sincethere is relatively little heat loss to other tube components such asthe dynodes 16, undesirable evaporation of antimony from the dynodes 16is avoided.

Electrical leads 44 of high temperature spring metal are connected tothe wafer 24 through the holes 30, 34 in the base of the wafer 24 afterit is mounted in the tube, as shown in FIG. 1.

GENERAL CONSIDERATIONS The invention has utility in various types ofelectron emissive tubes utilizing non-thermionic cathodes, and in whichit is desirable to avoid unnecessary heating of other internalcomponents of the tube when activating the cathode. Such non-thermioniccathodes are generally cesiated. The cathode can beaforward-biasedjunction emitter cathode, photocathode, or secondaryemitter. The semiconductor can be any semiconductor suitable for acathode layer. It may be, for instance, silicon or a compound or alloyfrom Groups lIlA and VA or IIB and VIA of the Periodic Chart of theElements.

The thickness of the ceramic wafer material is preferably great enoughto result in a relatively uniform temperature on the cathode side. Ifthe substrate wafer is too thick, however, there may be spalling of theceramic due to thermal stress. The pattern of the heater element can beany of various patterns which result in a relatively uniform heat outputover the surface. It is desirable, however, where the substrate waferiselongated, as in the preferred embodiment, to provide for additionalheat input to the substrate wafer near the ends of the substrate.

Various metals can be used for the heater pattern and for themetallizing on the emitter cathode face. Refractory metals, such asmolybdenum and tungsten are preferably used where the cathode layer mustbe grown on the substrate wafer directly by vapor deposition. This isdue to the severe conditions in the growth furnace for Ill-V compoundvapor phase deposition. During growth of the cathode layer, thesubstrate wafer is exposed to highly reactive gases at temperatures onthe order of 600C to 1,000C. Molybdenum and tungsten are the only metalsin general use which can withstand such conditions and which arecompatible with Ill-V compounds to the extent needed for growing a layerof sufficiently regular crystallinity for efficient performance of thecathode layer. Where the cathode layer is itself sufficiently conductiveto operate without a metallizing pad under it, the cathode layer may bedeposited directly on the ceramic.

Although the substrate wafer of the preferred embodiment is relativelyopaque aluminum oxide, other oxide ceramics such as sapphire and spine]may be used instead. The choice of ceramic is not critical. The ceramicmust be suitable for the vacuum conditions required in the finishedtube. The ceramic is preferably suitable for metallizing with themolybdenum or tungsten, capable of withstanding the temperaturesnecessary for forming the cathode layer and for activating the layerafter incorporation of the structure into a tube envelope. High aluminaceramics are especially suitable.

The heater pattern may be on the same side of the ceramic substratewafer as the cathode, and may be separated from the cathode layer by aninterposed electrically insulating layer such as silicon dioxide or bein direct physical contact with the cathode layer. Primarily, the heaterpattern is in direct thermally conductive contact with the cathodelayer. Direct thermally conductive contact means that the heat transferfrom the heater to the cathode layer is primarily by thermal conduction,either by direct physical contact, or through intermediate thermallyconducting material.

After the heater pattern is used for the cesiation processing of thecathode, it becomes a passive structure which is not used for operationof the tube, as the cesiated cathode is not heated for obtainingelectron emission.

We claim:

1. An electron emissive tube of the type comprising:

an evacuated envelope;

a non-thermionic, electron emissive cathode, having a photocathode layerand a work function reducing material on said photocathode layer, in theenvelope, and

a collector anode for electrons emitted from the electron emissivecathode,

wherein the improvement comprises:

a ceramic substrate on one surface of which the cathode is disposed, anda heater pattern of metallizing on the substrate in direct thermallyconducting contact with the cathode.

2. The tube defined inclaim 1 wherein the heater pattern of metallizingis on the surface of the substrate opposite the cathode.

3. The tube defined in claim 1 wherein the emissive cathode comprises:

-a metal layer on the one surface of the substrate;

a layer of semiconductor on the metal layer, and

a work-function-reducing layer comprising at least one member of thegroup consisting of cesium, oxygen, and fluorine on the surface of thesemiconductor layer.

4. The tube defined in claim 3 wherein the metal layer consistssubstantially of a metal from the group consisting of molybdenum andtungsten.

5. The tube defined in claim 4 wherein the semiconductor comprises atleast one element from Groups lIIA, VA, 11B, and VIA of the PeriodicChart of the Elements.

6. The tube defined in claim 1 wherein the heater pattern metallizingconsists substantially of a metal from the group consisting ofmolybdenum and tungsten.

7. An electron emissive tube in accordance with claim 1, characterizedin that the ceramic substrate has a pair of spaced aperturestherethrough and said heater pattern of metallizing extends over theceramic substrate between-the apertures.

8. An electron emissive tube in accordance with claim 7, characterizedin that the heater pattern of metallization comprises an electricallyresistive metal film of substantially uniform width extending in azig-zag pattern between said apertures, said metal film having a portionof narrower width respectively adjacent to each of said apertures.

I t 4' t

1. An electron emissive tube of the type comprising: an evacuatedenvelope; a non-thermionic, electron emissive cathode, having aphotocathode layer and a work function reducing material on saidphotocathode layer, in the envelope, and a collector anode for electronsemitted from the electron emissive cathode, wherein the improvementcomprises: a ceramic substrate on one surface of which the cathode isdisposed, and a heater pattern of metallizing on the substrate in directthermally conducting contact with the cathode.
 2. The tube defined inclaim 1 wherein the heater pattern of metallizing is on the surface ofthe substrate opposite the cathode.
 3. The tube defined in claim 1wherein the emissive cathode comprises: a metal layer on the one surfaceof the substrate; a layer of semiconductor on the metal layer, and awork-function-reducing layer comprising at least one member of the groupconsisting of cesium, oxygen, and fluorine on the surface of thesemiconductor layer.
 4. The tube defined in claim 3 wherein the metallayer consists substantially of a metal from the group consisting ofmolybdenum and tungsten.
 5. The tube defined in claim 4 wherein thesemiconductor comprises at least one element from Groups IIIA, VA, IIB,and VIA of the Periodic Chart of the Elements.
 6. The tube defined inclaim 1 wherein the heater pattern metallizing consists substantially ofa metal from the group consisting of molybdenum and tungsten.
 7. Anelectron emissive tube in accordance with claim 1, characterized in thatthe ceramic substrate has a pair of spaced apertures therethrough andsaid heater pattern of metallizing extends over the ceramic substratebetween the apertures.
 8. An electron emissive tube in accordance withclaim 7, characterized in that the heater pattern of metallizationcomprises an electrically resistive metal film of substantially uniformwidth extending in a zig-zag pattern between said apertures, said metalfilm having a portion of narrower width respectively adjacent to each ofsaid apertures.